Multi-parametric Observation of Volcanic Lightning at Sakurajima Volcano, Japan.

Abstract:

Ash-rich volcanic plumes are very often associated with electrical discharges producing majestic display of lightning. Observation and understanding of this phenomenon can shed light on crucial properties of the plume such as mass eruption rate and content of fine particles, as recently demonstrated by laboratory investigation of volcanic lightning (Cimarelli et al., 2014). Despite the recent advances in experimental investigations and the increasing detailed observation by lightning monitoring arrays, many fundamental questions are yet unsolved. In particular, to which extent electrical discharges in volcanic plumes are comparable to thundercloud lightning? Is the presence of hydrometeors in the plume a necessary condition for the generation of volcanic lightning? Multiparametric observation of electrical activity at erupting volcanoes is key to answering these questions. Here we present the results of a campaign of measurements conducted at Sakurajima volcano (southern Japan) where, for the first time, we combined synchronized high-speed imaging with magnetotelluric (MT) and acoustic measurements of ash-rich plumes generating electrical discharges and compare our observations with maximum plume height measurement and atmospheric soundings. Results show that flashes concentrate within the plume and closer to the crater. Good correlation if found between cloud-to-ground versus intra-cloud events and the frequency and duration of recorded MT-signals, while measured currents at Sakurajima are 10 to 100 times smaller than those produced by thundercloud discharges. Finally, atmospheric soundings show that plumes producing flashes didn’t cross isotherms relevant for ice formation thus discarding a relevant contribution of hydrometeors in the generation of the observed volcanic lightning.

Cimarelli et al. 2014. Experimenal generation of volcanic lightning. Geology v. 42, no. 1 doi: 10.1130/G34802.1